Automatic volume control circuit



L. T. FOWLER AUTOMATIC VOLUME CONTROL CIRCUIT Sept.l 7, 1937.

Filed-Dec. 16, 1933 1,0V tubes.

PatentedI Sept. '7, 1937 PATENT OFFICE AUTOMATIC VOLUME CONTRQL CIRCUIT Lester T. Fowler, Oaklyn, N. J., assignor to Radio Corporation of America, a. corporation of Dela- Ware Application December 16, 1933, Serial No. 702,676

7 Claims.

My present invention relates to amplification control circuits, and more particularly to a novel and improved type of delayed automatic volume control arrangement for; a radio receiver.

1n the past delay action has been secured in radio receivers employing automatcvolume` controlaarrangements by utilizing the self-biasing resistor in the cathode circuit of the controlled This self-biasing resistor supplied the initial grid bias for the controlled tubes when no bias voltage was being supplied from the automatic volume `control tube. The self-biasing resistor was of such magnitude as to supply `the 15` normal grid bias for the controlled tubes at the voltages used, and to supply a degree of delayed automatic volume control action. Such delayed automatic volume control action occurred when a signal caused the automatic volume control to 2Q start operating and supply a negative control voltage to the grids of the controlled tubes, the platecurrent of said `tubes then decreasing. 'Ihis resulted in a decrease in Voltage drop across the self-biasing resistors in the cathode circuits of 2 5` the controlled tubes. Since this decrease in volt- 4age drop was in a direction to initially oppose theaction of the automatic volume control tube, there was thus supplied a delayed action for the automatic volume control. As soon as the 30 voltage from the automatic volume control tube r"overcame the negative bias supplied by the selfb'iasing resistor, the automatic gain control action commenced in normal fashion.

Although such prior `arrangements were of value, since existing circuit elements could be utilized to accomplish the delayed automatic volume control action, it was recognizedthat the delay action of the automatic volume control could be improved by increasing the value of the 4,0.self-biasing resistor in the cathode circuits of 5rangement is described, for example, by A. W.

WBarber in U. S. Patent No. 1,903,542, issued April 1l, 1933, and is generically claimed therein. In the aforesaid Barber patent the bucking positive potential is specifically derived from an auxiliary 5q battery.

It is one of the main objects of my present invention to provide an improved type of delayed automatic volume control receiver wherein the self-biasing resistor in the cathode circuit of a ,controlled amplifier is utilized to secure the delay (Cl. Z50- 20) action, the bias resistor being of a'n abnormallyv high value for this purpose, and the excess negative bias voltage developed by the self-bias resistor being reduced by supplying a positive Voltage to the grid of the controlled tube without utilizing any auxiliary batteries.

Another important object of the present invention is to provide an automatic volume control arrangement for a radio receiver wherein the action of the volume control arrangement is delayed until the signals impressed upon the receiver assume a predetermined intensity, the delay action being secured by utilizing a self-bias resistor in the cathode circuit of at least one high frequency amplier, the self-bias resistor having a magnitude which is abnormally high, the excess negative bias developed across said self-bias resistor being compensated for by a positive grid potential developed across a second resistor disposed in the direct current path between the controlled amplier and the negative side of the aforementioned self-bias resistor.

Still other objects of the present invention are to improve generally the simplicity and efficiency of delayed automatic volume control arrangements for radio receivers, and especially to provide an arrangement of this type which is not only reliable in operation, but economically manufactured and assembled in a radio receiver.

The novel features which I believe to be Y characteristic of my invention are set forth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, will best be. understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown a conventional type of superheterodyne receiver which embodies one form of the present invention. This receiver is of a type Well known to those skilled in the art, and for this reason only such circuit elements will be described which are essential to a clear understanding of the invention. The receiver comprises the usual signal collector device, such as a grounded antenna circuit A, the latter being coupled to the tunable input circuit of the radio frequency amplifier tube l, which may be of the screen grid type. The numeral 2 designates an intermediate frequency amplifier tube which has a resonant input circuit tuned to the operating intermediate frequency. The numeral 3 designates any well known type of frequency changing network which is customarily utilized between the radio frequency amplifier of a superheterodyne receiver and the intermediate frequency amplifier. For the sake of simplicity of description, the network 3 is shown as of the composite first detector-local oscillator type. The numerals i and 5 designate the tunable signal circuits of the radio frequency amplifier and the frequency changer networks respectively, while the numeral 6 designates the tunable local oscillator circuit of the network 3. The dotted line 'u' is to be understood as representing any well known type of mechanical uni-control which can be used to adjust the variable tuning condensers of the tuned circuits 4, 5 and 6.

The electron discharge tube 8 is a multiple duty tube known as a duplexediode triode, and its associated circuits, as well as the specic construction of the tube, have been described and claimed by T. M. Shrader, in his application Serial No. 622,140, filed July l2, 1932, Patent No. 2,058,834, October 27, 1936. For the purpose of the present application it is sufficient to point out that such a tube comprises a pair of diode sections and a triode section, a common cathode being utilized for these three sections. In the present case the two diode anodes are strapped together and cooperate with the common cathode to provide a diode rectier circuit whose tuned input circuit 9 is resonant to the operating intermediate frequency and is coupled to the tuned output circuit I of the intermediate frequency amplifier tube. The resistor I I is disposed in the diode rectifier circuit between the common cathode of tube 3 and the low alternating current potential side of tuned circuit 9, the resistor II being shunted by a radio frequency signal by-pass condenser.

The triode section of tube 8 functions as an audio frequency amplifier, and the signal grid thereof may be connected to the negative side of resistor Il through a radio frequency choke coil I 2, a radio frequency by-pass condenser being connected between the cathode of tube 8 and the grid side of choke I 2. The grid of tube 8 has impressed upon it the audio frequency component of the rectified signal currents, the grid also being diode biased by virtue of its connection to the negative side of the resi-stor II. The amplied audio frequency potential component flowing in the triode section of tube 8 is transmitted to a succeeding audio frequency amplifier network, for amplification and utilization by a reproducer, through a coupling condenser I3.

The anode of the triode section of tube is connected for direct current potential to the receiver potential supply source B (which is not shown to preserve simplicity of disclosure), the space current path of the triode section of tube 8 including a resistor R2, this resistor being shown connected between the cathode of the tube and ground. It will be understood, of course, that the ground side of the resistor R2 will connect back to the minus terminal of the' potential supply source B. The anode circuits of tubes I and 2 are shown connected to the potential supply source B, and here again it will be clearly understood that the cathode circuits of the tubes I and 2 return to the negative sides of these potential supply sources. Of course, a common potential supply source may be used for tubes I, 2 and 8. Appropriate radio frequency by-pass condensers are utilized in tubes I, 2 and 8 at points well known to those skilled in the art.

The resistor R1 is a self-bias resistor for the aoeasos' signal grids of tubesI and 2, this resistor being connected between ground and the cathode of tube I. The lead I4 connects the cathode of tube 2 to the cathode side of resistor R1. It will, therefore, be seen that the resistor R1 is disposed in the space current paths of each of amplifier tubes I and 2. It is within the scope of the present invention to employ a self-bias resistor in the cathode circuit of amplifier tube 2 which is independent of the self-bias resistor in the cathode circuit of tube I. The signal grid of amplier tube I is connected to the negative, or ground, side of resistor R1 through a path which includes the lead I5, the resistor I I and the resistor R2, the signal grid of amplifier tube 2 being connected to the lead I5 through a lead I5. The lead I5 functions as the automatic volume control lead, and for this reason is designated by the letters AVC. The leads I5' and I5 include resistors for suppressing the audio frequency ripples in the automatic volume control connection, and the reference numerals I6 and I6 designate such ripple suppressor resistors.

As the signal energy impressed upon the diode detector tuned circuit 9 increases, the point II of resistor II increases in negative direct current potential, and, therefore, the signal grids of controlled amplifiers I and 2 become more negative in direct current potential with respect to their respective cathodes. This results in a decrease in the gain of the controlled amplifiers, the reverse action taking place when the signal intensity decreases from maximum intensity towards minimum intensity. The sensitivity of the controlled amplifiers I and 2 are a maximum when no signals are impressed upon the receiver. It is desirable to delay the automatic Volume control action until signals of a predetermined intensity level are impressed upon the receiver. The reasons for such delay action are too well known to those skilled in the art to require an explanation. As stated heretofore, the self-bias resistor of the controlled ampliers supplied a degree of such delay action in receivers utilizing automatic volume control in the past. In the aforesaid Barber patent, however, it was shown how such simple delay action could be greatly improved by increasing the value of the self-bias resistor in the cathode circuits of the controlled amplifiers, and bucking out the excess negative bias developed because of this increased resistor value by using a positive potential derived from an auxiliary voltage source.

In the present arrangement the need for such an auxiliary battery is entirely eliminated, and the improved delay action secured. It will be observed that the signal grid of each of the controlled amplifiers I and 2 is connected to the negative side of the self-bias resistor R1 through a path which includes the resistor R2. In effect the signal grids of the controlled amplieis are connected to the positive side of the resistor R2, and for this reason the potential drop across the resistor R2 is opposed to the negative grid biasing voltage developed across resistor R1.

Since this bucking positive potential is secured by merely disposing a resistor of appropriate value in the cathode circuit of the triode section of the tube 8, it will be seen that the excessive negative biassing potential developed across the self-bias resistor has been eliminated in a simple and effective manner. The values of the resistors R1 and R2 will depend upon the amount of delay action desired in the automatic volume control arrangement. In an actual embodiment of the invention the resistor R1 had avalue of the order of 850 to 1000 ohms, and the value of the resistor R2 was chosen so as to provide a compensating positive potential sufficient i to bring the effective grid bias potential for tubesl I and 2 down to the value which will be supplied by the usual 200 ohm self-bias resistor. It will be observed that the tube 8 functions as a combined second detector, automatic volume control rectifier and audio frequency amplifier. 'I'he di-` ode circuit which includes the tuned circuit 9 functions as the second detector as wellas the automatic volume control rectifier.

With regard to specific voltages, and assuming that a normal negative bias of -3 volts is required on the signal grids of tubes I and 2, the controlled amplifiers, the value of resistor R1 may be sufficiently high to develop a negative potential of -10 volts on the signal grids of tubes I and 2. In that case, the value of resistor R2 will be so chosen that the cathode of tube 8 is raised '7 volts above ground, and for this reason the signal grids of the controlled amplifiers will have an effective normal bias of -3 volts. It will, therefore, be seen that in the present invention the self-bias resistor is deliberately made of an abnormally high value, and that the excess negative grid bias voltage which results is eliminated by utilizing a compensating positive potential which is impressed upon the signal grids of the controlled amplifiers, this compensating positive potential being developed across a resistor disposed in the space current path of the audio amplifier network.

The actual bias on the controlled amplifier grids is the difference between the voltage drop across the resistor R1 and that across the resistor Rz (when no signal is being received and there is no drop across resistor II). When the automatic volume control arrangement commences to function, the plate current of the controlled amplifiers decreases, but since the drop across the biasing resistor R1 is initially greater than in the case where the self-biasing resistor is of normal value, the action of the automatic volume control is delayed further.

A further detailed explanation of the delay feature is as follows: First assume the case where no signal is being received. No current is fiowing in the diode circuit so there is no voltage drop across resistor II. Hence, the bias on the controlled tubes is the difference between the drops across R1 and R2, and the value of these resistors is so adjusted that the bias is correct to give maximum sensitivity with the voltages used. Now when a signal is received, rectification occurs inthe diode circuit and automatic volume control voltage appears across resistor II. This voltage is in a direction to bias down the controlled tubes. This action reduces the space current of the controlled tubes flowing through R1. which reduces the voltages across R1. This reduction in voltage across R1 is the important point, since it is in a direction to oppose the action of the automatic volume control voltage across resistor II. The voltage across resistor I I is tending to bias the tubes off and reduce the amplification of the controlled tubes, but the decreasing voltage across R1 due to this action is tending to reduce the negative bias of the controlled tubes and to hold the amplification nearly constant. Therefore, a valuable amount of delay takes place as long as the voltage across R1 is appreciable with respect to the voltage across resistor II.

Now when a very strong signal is tuned in, the voltage across resistor II rises to a relatively largevalue, andthe space current of the'controlled tubes is reduced to such an extent that the voltage drop acrossRrbecomes very small. After this happensno .further delay takes place because the drop across R1- is substantially zero insofar as `the automatic volume control action is concerned. The space current of tube t is fed through a high resistance `from the B+ terminal of the plate supplyxsource. This resistance is several times the tube impedance, and so is largely instrumental in .determining the space current fiow of tube 8. Therefore, the bias voltage of tube Ii may vary over a fairly wide range before the space current israppreciably affected. Of course, if enough negative bias is applied to the grid of tube B, its space current is eventually cut off. However, the present delayed AVC action is concerned only with the first few volts appearing across resistor II, that is to say, when it is desired that the delay action take place. Accordingly, with the space current of tube 8 remaining practically constant for the iirst few volts appearing across resistor I I, the voltage drop across resistor -Rz will .remain practically constant, and the delay action described above will take place.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications maybe made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:-

1. A radio receiver including a high frequency amplifier provided with a tunable input circuit, a self bias resistor in the cathode circuit of said amplifier, said resistor having an abnormally high magnitude whereby it develops excessive negative grid bias for the amplifier, a rectifier coupled to the amplifier for producing a direct current voltage from amplified signal energy, a resistor in the rectifier circuit for developing said voltage, an automatic volume control connection between the rectifier resistor and the amplifier grid, an electron discharge device having a grid connected to a point on the rectifier resistor, a third resistor in the space current path of' said device, and connections between the first resistor and the third resistor whereby the voltage developed across said third resistor is opposed in sign to the said excessive grid bias.

2. A radio receiver including a high frequency amplifier provided with a tunable input circuit, a self 4bias resistor in the cathode circuit of said amplifier, said resistor having an abnormally high magnitude whereby it develops excessive negative grid bias for the amplifier, a rectifier coupled to the amplifier for producing a direct current voltage from amplified signal energy, a resistor in the rectifier circuit for developing said voltage, an automatic volume control connection between the rectifier resistor and the amplifier grid, an electron discharge device having its grid connected to a point on the rectifier resistor, a third resistor in the space current path of said device, means for connecting the first and third resistors in such a manner that the voltage across the third resistor is opposed in sign to said grid bias, the voltage developed across said third resistor being of a lesser magnitude than said grid bias.

3. In a carrier wave receiver, the combination with a carrier wave transmission tube, a diode rectifier having input and output impedances associated therewith, means coupling the transmission tube output circuit to the input impedance of said diode rectifier, and circuit elements for transferring back to said transmission tube a. gain control voltage developed across the output impedance of said diode rectier, of a control grid bias resistor in said tube cathode circuit of such magnitude as substantiallyv to postpone reduction in transmission tube gain until said gain control Voltage reaches a predetermined value, means including serially in a direct current path having the diode cathode as an element thereof for developing a voltage of lesser magnitude thanthe voltage across the bias resistor, and connections between the bias resistor and the last means whereby the lesser voltage is opposed to said bias voltage.

4. An electrical wave transmission system of the type including an amplier and a rectifier for impressing upon said amplier a control grid bias potential varying automatically with changes in the amplitude of the electrical waves impressed upon said amplifier, characterized by the fact that the cathode circuit of said ampli er, includes a bias resistor acro'ss which the space current ow establishes a bias potential, said resistor having a magnitude such that for a predetermined range of amplier input voltages the changes in cathode bias potential substantially offset changes in the rectier bias potential, an audio amplifier having its input electrodes coupled to said rectifier, an impedance in the space current of the audio amplier, and connections between the impedance and the bias resistor such that voltages across the two are in opposition.

5. In a receiver as defined in claim 1, the grid of said amplier being connected to` said bias resistor through a path including said automatic volume control connection and the third resistor.

6. In a receiver as defined in claim 1, said rectier and device being included in a common tube envelope and the electrodes thereof having a common cathode.

'7. In a receiver as dened in claim 2, said device comprising an audio amplier, and the electrodes of the rectifier and audio amplifier being disposed in a common tube envelope.

LESTER T. FOWLER. 

